It doesn't matter if you're not the only one. Somebody did the math and determined that people without mobile numbers weren't worth the support cost. It sucks to be the wrong side of that math but it happens to a lot of people for a lot of different things.
Developers tend to try and include everyone but that is often a mistake -- the benefit rarely outweighs the cost.
> Somebody did the math and determined that people without mobile numbers weren't worth the support cost.
While I'd normally agree I'm not sure that Yahoos decision making process should be the role model regarding optimizing user growth. Their lack of users was not a minor issue.
Calculation is "make mobile phone number optional and have most of the users refuse to give it away because they know better and miss profit opportunity or make it mandatory and risk having a few fringe users use worthless numbers", so optimal profit per user is the chosen way when the company is driven by maximizing their own profit (actually shareholders) and do not care about people, communities and such.
Developers tend to try and include everyone but that is often a mistake -- the benefit rarely outweighs the cost.